Most organic polymers, especially polyolefins such as polyethylene or polypropylene, which are used for manufacturing articles tend to develop color with passage of time. The discoloration may be due to various factors such as the decomposition of the polymer, the antioxidant or to the presence of residual metal catalyst or residual peroxides used to control polymerization. It may also be due to the basic nature of polymeric material, the additives used in the polymeric material or the effect of heat on the polymeric material. The development of color is undesirable because it indicates polymer degradation and results in further reduction of the quality of the polymer. Thus, one of the objects of this invention is to provide a stabilized composition which would improve the quality of polymers by preventing the discoloration of the polymer.
The prior art discloses many materials which inhibit the discoloration of polymers on exposure to heat and light. It is well known to stabilize polyolefins and styrenic polymers by the use of phosphites. Thus, for example, phosphites can be used in conjunction with UV absorbers such as benzophenones and benzotriazoles to achieve improved stabilization of polypropylene on outdoor exposure to sunlight. However, a phosphite is generally not used as the sole light stabilizer in polypropylene because of its limited effectiveness. Unexpectedly, it has been found that the phosphites of this invention can be used effectively in polymers, such as polypropylene, as the sole light stabilizer. Furthermore, it has also been surprisingly found that these phosphites act as effective antioxidants for polymers, both when used alone and in combination with thiosynergists.
It is also well known in the art to use phosphites in conjunction with hindered phenols as stabilizers for unsaturated elastomers. A notable example of a phosphite widely used in such a combination is trinonylphenyl phosphite (TNPP). It has now been found that the phosphites of this invention are surprisingly much more effective color stabilizers and antioxidants than trinonylphenyl phosphite.